1. Field of the Invention
The present invention relates to (Co+Cu)/Cu superlattices for use in giant magnetoresistance (GMR) material. More particularly, the present invention relates to a Co-Cu/Cu superlattice in which the Co+Cu layers consist of codeposited Co and Cu.
2. Description of the Related Art.
GMR and materials exhibiting GMR have been used for detecting the magnetic field in magnetic sensors, magnetic heads for computer disks, rotation detectors, position detectors and other types of sensors. The phenomenon of GMR was first identified for multilayer thin films that consisted of thin layers of the ferromagnetic metal Fe separated by a nonferromagnetic Cr metal spacer as described in the paper by M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff. P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas, Phy. Rev. Lett. 61, (21), 2471, Nov. 21, 1988. Although the magnetoresistance in the Fe/Cr multilayers was not known at that time to be an oscillating function of the Cr thickness, the maximum magnetoresistance was obtained with a Cr thickness of 9 .ANG.. This was subsequently shown to be approximately at the first maximum of an oscillating dependence on Cr thickness as described in the paper by S. S. P. Parkin, N. More, and K. P. Roche, Phys. Rev. Lett. 64, (19), 2304, May 7, 1990. A similar oscillating behavior was found with Co/Ru. Later it was found that layers of ferromagnetic Co with Cu spacer layers also showed the effect and that this combination of metals gave a large change of resistivity with a relatively small change of the magnetic field D. H. Mosca, F. Petroff, A Fert, P. A. Schroeder, W. F. Pratt Jr., and R. Loloee, J. Magn. Magn. Mater. 94, L1-L5, Mar. 1, 1991; S. S. P. Parkin, R. Bhadra, and K. P. Roche, Phys. Rev. Lett. 66, (16), 2151, Apr. 22, 1991. The magnetoresistance in this system is an oscillatory function of the thickness of the Cu layers. Maxima that occur with Cu layer thicknesses near 9 .ANG. and near 20 .ANG. are particularly useful. These conditions are commonly referred to as the first and second anti-ferromagnetic maximum (AFM), respectively. In practice, the second AFM gives a larger rate of change of resistivity with change of magnetic field and is, therefore, usually preferred. Typical values of the Co layer thickness are of the order of 10-15 .ANG..
The physical origin of the phenomenon is the interaction between consecutive Co layers via the Cu spacer layers. With appropriate choice of the Cu spacer thickness (i.e. at an AFM) and in the absence of an applied magnetic field, the magnetizations of adjacent Co layers are aligned antiparallel in consequence of an antiferromagnetic coupling force. This is a relatively high-resistance state. With the application of a magnetic field (preferably in the plane of the film) the magnetizations of the Co layers are made parallel and the resistance decreases. The change in resistivity is largest at the first AFM, but this typically requires a magnetic field of 1-2 kOe. At the second AFM, the field required to overcome the anti-ferromagnetic alignment is substantially smaller so that this condition gives the maximum sensitivity to change of the magnetic field.
A disadvantage of the GMR phenomenon in practical applications is the occurrence of hysteresis. When a material exhibits hysteresis, the maximum resistance does not occur exactly at zero applied field. Instead, the resistance lags behind the applied field. This causes an undesirable uncertainty in the magnetic field that is associated with a particular value of the resistance. As a fraction of the magnetic field required for saturation, the splitting of the Resistivity/Magnetic (RIM) field curve is larger at the second AFM than at the first.
U.S. Pat. No. 5,341,118 ('118), issued Aug. 23, 1994, teaches the use of alternating layers of magnetic and nonmagnetic metals in which the magnetoresistance is an oscillatory function of the thickness of the nonmagnetic metal to detect a magnetic field or changes therein. The combinations that are specified include Co/Cu. However, neither it nor the other art cited addresses the problem of reducing magnetoresistive hysteresis that is described in the '118 patent. FIG. 14 of the '118 patent shows that substantial magnetoresistive hysteresis was seen near room temperature in the Co/Cu multilayers.
It was found that the magnetoresistive hysteresis of Co/Cu multilayers could be reduced by making the Co layers very thin (&lt;4 .ANG.) as discussed in Giant magnetoresistance in Co/Cu multilayer with very thin Co layers: Reduced hysteresis at the second antiferromagnetic maximum. D. J. Kubinski and H. Holloway, J. Appl. Phys. 79 (3), Feb. 1, 1996, and incorporated herein by reference. The physical state of the thin Co layers is not well defined and is specified by an average thickness. The actual thickness is physically limited to multiples of discrete spacings of the crystal lattice of Co, which are of the order of 2 .ANG.. Thus, the average thicknesses of the very thin polycrystalline Co layers, which are of the order of 3 .ANG., arises from closely intermixed regions with significantly varied thicknesses. While not wishing to be bound to this theory, it is believed that the very thin Co layers are broken up into closely spaced isolated islands.
It was found that the hysteresis is greatly reduced when the average Co thickness is less than 6 .ANG. and that it is nearly eliminated with an average Co thickness of 3 .ANG.. With the 3 .ANG. thick Co layers, the magnetoresistance is somewhat smaller than with 15 .ANG. thick Co layers, but this is substantially offset by a narrower peak in the Resistivity/Magnetic field curve. Consequently, the sensitivity to changes in the magnetic field remains large. The maximum slope of the Resistivity/Magnetic field curve gives a sensitivity of: EQU S=(l/R)(dR/dH).apprxeq.0.7.times.10.sup.-3 Oe.sup.-1
for the 3 .ANG. thick Co layers, compared with: EQU S.apprxeq.1.2.times.10.sup.-3 Oe.sup.-1
for the 15 .ANG. thick Co layers. With further reduction of the average Co thickness to 1 .ANG. the magnetoresistance is decreased to less than 1%.
It was also found that hysteresis can be reduced in the superlattices (particularly of Co/Cu) in which some of the ferromagnetic layers (e.g. Co) are made very thin. In Giant magnetoresistance in Co/Cu multilayers with Co layers of alternating thicknesses: Reduction of magnetoresistive hysteresis. H. Holloway and D. J. Kubinski, J. Appl. Phys. 79 (1), May 1, 1996, incorporated herein by reference, the authors describe the magnetoresistive properties of Co/Cu multilayers in which the thicknesses of anti-ferromagnetically coupled, Co layers are alternated between thicker and thinner values. In such structures the thicker Co layers become coupled more strongly to applied magnetic fields than are the thinner Co layers. Such structures combine the large magnetoresistance of Co layers with conventional thickness of .about.15 .ANG. with the small hysteresis of Co layers that have thicknesses .about.3 .ANG..
In an article by the inventors of the present invention, Giant magnetoresistance in Co.sub.1-x Cu.sub.x /Cu multilayers: A new approach to reduced magnetoresistive hysteresis, D. J. Kubinski and H. Holloway, to be published in J. Appl. Phys. 82, (1) Jul. 1, 1997, and incorporated herein by reference, the authors describe the properties of the multilayers incorporating Co+Cu alloy layers that are the subject of this invention. As will be shown, this provides a new and useful method for obtaining giant magnetoresistance with greatly reduced magnetoresistive hysteresis.